Salt Mining Question

Hello.

I tried to research this on the Web, but I'm having trouble finding
the answer to my rather specific questions, so I thought I'd see if
anyone in here could help me.

What I am concerned with specifically is salt mining as opposed to
saltpeter mining.

I know that they are not the same thing, but my impression from what I
have read is that they often occur naturally in the same kinds of
places, and are therefore often mined together.

Is this correct, or am I totally off base?

I'm also curious to know whether it would have been possible in "olden
days" to have mistaken one for the other.

Thanks in advance for your help.

-- 
Leigh Butler